A group of galactic fossils blaze a bright blue in the X-ray region in this view from a European observatory. 

The European Space Agency’s XMM-Newton X-ray Telescope caught this image of a galactic cluster known by the hefty moniker RX J1416.4+2315.

Astronomers have dubbed the galaxies within this cluster, whose dominant elliptical galaxy sits about 500 million light-years from Earth and shines 500 thousand million times brighter than the Sun, as fossils because researchers aren’t sure how they’ve had time to form due to their age and mass. The title is also due to the large central galaxy, since clusters are called fossils if their member galaxies have merged into a single giant object due to the gravitational influence of dark matter, which is believed to be the case here, researchers said.

RX J1416.4+2315 is the most massive and hottest known fossil galaxy group. Observations from XMM-Newton and the Chandra X-Ray Observatory, as well as optical and infrared data, found that the fossil group sits within a halo of gas that stretches across three million light-years and is heated to a temperature of about 50 million degrees.

Of the 300 solar masses contained in the cluster, only two percent is from stars and galaxies, with 15 percent more in the form of hot, X-ray emitting gas. The rest of the mass appears to come from dark matter, researchers said.

Current projections state that the fossil group should not have had enough time to form given the age of the universe, which is about 13.7 billion years old. Researchers hope by studying the cluster, they will shed new light on how formation of structures in the early universe.

-- SPACE.com Staff

Credits: Khosroshahi, Maughan, Ponman, Jones, ESA, ING